Foods and other goods are frequently packed in rigid-walled containers which, as early as during production, are provided with a label by the “in-mould labelling” or “IML method”. In this method, a label is laid, usually by a robot, in the open mould in such a way that the printed (out)side of the label is in contact with the mould wall, while the unprinted (inside) faces the container to be shaped. Flat, fold-free lying of the label in the mould is achieved, for example, by means of a vacuum applied to fine air-removal perforations, with the perforations being substantially sealed by the label. Alternatively, electrostatic forces between the electrostatically charged label and the earthed mould ensure flat lying of the label.
The advantage of the IML method is that the print quality of the separately produced labels is significantly better than direct printing of the containers. Application of the label during shaping of the container is inexpensive and effective. There is no need for adhesives, coated backing films or papers and it is therefore not necessary to dispose of waste and residues. The in-mould labelling method can be combined with various container production processes. Variants of the IML method have been proposed or already developed for, for example, injection moulding, thin-wall injection moulding, blow moulding and thermoforming and the injection stretch blow moulding of containers.
Furthermore, methods for labelling containers after their production or after their filling are known, such as, for example, wrap-around labelling, patch labelling, self-adhesive labels and thermolabelling.
Thermolabelling covers all methods in which a label is applied to a container under the action of heat. For example, the pre-shaped container can be heated in a suitable process and the label pressed onto the container wall by means of pressure or with the aid of brushes or rolls in such a way that it is subsequently firmly bonded to the container.
In both in-mould labelling and thermolabelling, bubble-free application of the label is an important and at the same time problematic requirement. In order to achieve this, the inside surface of the label film, i.e. the one facing the container, is frequently structured or roughened to simplify exit of air from the gap between label and container wall.
In the case of simple label shapes, the label is usually supplied in roll form and cut to size on the machine in which the container is shaped (cut in place). This method is particularly appropriate in the case of simple, for example rectangular, label shapes, as are used, in particular, in wrap-around labelling (WAL) and patch labelling of essentially cylindrical containers and bottles.
In the case of more complex outlines, the label is frequently cut to size in advance, stacked in magazines, later removed from the stack at the labelling machine and laid in the mould (cut & stack method). The labels here are firstly printed, for example, by the so-called sheet offset method or other suitable methods and cut to their final shape directly after the printing process. Both the unprinted and the printed sheets and labels must be readily processable in the individual process steps of sheet cutting, printing, label cutting and feeding to labelling. The sheet and the label are stacked and unstacked. In the process, the films must slide easily against one another and they must not become electrostatically charged. Besides providing the label film with lubricants and antistatics, corresponding structuring of the film surface is therefore necessary.
For the production of the containers, use is made of various materials, such as, for example, polypropylene (PP), high- or low-density polyethylene (HD-PE or LD-PE or LLD-PE, polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate PET; polycarbonate (PC) and in individual cases also mixtures of such polymers. This gives rise to various requirements of the label materials with respect to strength, extensibility, rigidity, gloss/mattness and adhesion to the container wall. For cost reasons, biaxially oriented polypropylene films (BOPP films) are preferably employed, these applications currently being restricted to containers made from polypropylene and polyethylene.
The conventional BOPP labels adhere, even at elevated temperatures, poorly to the other container materials mentioned. It is not possible to apply labels made from BOPP film to containers made from PET, PS, PC or PVC by means of in-mould or thermolabelling methods. In particular, it has hitherto not been possible to use BOPP films in the injection-moulding IML method on polystyrene. It has been just as unsuccessful to employ label films made from BOPP films in the thermolabelling method on PET bottles. Here, corresponding adhesives have to be applied for attaching the label. In spite of extensive attempts to combine various container materials and various label materials with the diverse known labelling methods, the choice of successful combinations remains very restricted. The direct use of BOPP films in the IML or thermolabelling method for the labelling of containers which do not consist of PP or PE has hitherto not been-possible. For the adhesion of BOPP labels to PS, PET, PC, PVC, etc., adhesives, coatings, lacquers or similar aids are necessary, it being necessary to apply these subsequently, i.e. after production of the film, in an additionally processing step. This firstly makes the label more expensive and sometimes has a disadvantageous effect on the other service properties.
British Application GB 2 223 446 discloses a BOPP film which consists of at least two layers, with the comparatively thinner layer consisting of a blend of a material which has low seal seam strength to PVDC and a material which consists of a copolymer of an alkene and an unsaturated monobasic acid or ester thereof. In preferred embodiments, suitable materials having low seal seam strength to PVDC are high- and low-density polyethylenes, and suitable copolymers are those of ethylene with acrylic acid esters, where, in particularly preferred embodiments, these copolymers can comprise unsaturated dibasic acids or anhydrides thereof, such as, for example, maleic anhydride, as further monomers. Corresponding copolymers and terpolymers have been described in EP 0 065 898.
On repetition of British Application GB 2 223 446, it was observed that the process described therein results, on use of the formulations indicated therein, in considerable deposits on the heating and stretching rolls of the longitudinal stretching unit of a sequential BOPP machine which are unacceptable for industrial practice. Variations within the limits of the disclosed teaching brought about no advantage or only a slight advantage with respect to the amount and speed of the roll coating built up, or other service properties were adversely affected.
The object of the present invention was to provide a biaxially oriented polyolefin film which is inexpensive and can be employed in a diverse manner as label film. The film should, in particular, be applicable by the IML or thermolabelling method to containers made from various materials, such as, for example, PP, PE PET, PS, PC, PVC, etc. and should have good adhesion. It is of course necessary that the film can be produced without the formation of deposits on the rolls of the longitudinal stretching unit. In addition, the other important service properties and the appearance of the film, or of the label produced therefrom, should not be adversely affected. In particular, the film should be printable on one side and should be readily stackable and destackable in the processing process.